You must reply to Dennis's letter soon.

你必须赶快给丹尼斯回信。

youdao

If you do receive a letter offering you the job, you must reply by letter (keep a photocopy) as soon as possible.

如果你真的收到一封信，让你去工作，你应该赶快写封回信。

youdao

When someone says "thank you," you must reply "you're welcome." you must! Silence can be rude, and dangerous.

当有人说“谢谢”时，你应该回答“不客气”。你必须这样说。不回答或保持沉默是非常粗鲁的行为，而且也是危险的行为。

youdao

We look at what emails you reply to if you reply in theory that suggests this user must care about whoever sent this email or the topic they are in.

我们看看哪些邮件要回复,如果你回复了,理论上来说这表明了用户一定很关心,发件人以及邮件的主题。

哈佛公开课 - 计算机科学课程节选

You must then reply, confirming that you weren't the victim of a prank and do indeed want the information being sent.

FORBES: Spam Warfare